The main function of the packages is the preservation and protection of all types of products, with foods and raw materials the field of highest priority. These products require attention, given the contamination generated by microorganisms (bacteria, spores, fungi, etc.) during manipulation (Tharanathan, 2003). Protection is made through packages, which are generally elaborated from synthetic polymers. Nevertheless, the indiscriminate use of synthetic packages has generated serious ecological problems, contributing to the environmental contamination provoked by solid wastes of low degradability, which has driven the search for natural biopolymers. Availing of natural resources as source of conservation and recycling becomes an excellent option and innovation in the development of new biodegradable products. Its total biodegradation in products like CO2, water and subsequently in compost it is a great advantage against synthetic materials (Bastioli, 2001).
Total replacement of synthetic plastics by biodegradable materials for the elaboration of packages has not been accomplished until now; nevertheless, some synthetic polymers have been substituted by other natural polymers, in specific applications. Said replacements have permitted the development of products with specific characteristics related to the barrier, mechanical, and thermal properties in certain packages like films, protectors, foams, wrappings, plates, cups, spoons, bags, etc., (Avérous and Boquillon, 2004; Wang et al., 2003).
Natural biopolymers come from four big sources: animal origin (collagen/gelatin), marine origin (chitin/chitosan), agricultural origin (lipids and fats and hydrocolloids: proteins and polysaccharides), and microbial origin (polylactic acid (PLA) and polyhydroxyalkanoates (PHA)) (Tharanathan, 2003).
Polysaccharides are known for their complex structure and functional diversity (Stawaski and Jantas, 2003). The linear structure present in the cellulose (1,4-b-D-glucan), amylose (a component of starch 1,4-a-D-glucan), and chitosan (1,4-b-D-carbohydrate polymer) provide the films hardness, flexibility, and transparency; the films are resistant to fats and oils.
Starch is an abundant raw material, specifically that coming from corn; it has thermoplastic properties when the structural disruption takes place at molecular level. The presence of amylose at 70% in amylose-corn starches gives a strong structure and more flexibility to the film. The branched structure of the amylopectin generally gives the film poor mechanical properties. Compounds from hydroxypropylated starches are used for the preservation of candies, raisins, nuts, and dates to avoid oxidative rancidity (Tharanathan, 2003). Synthesis of the copolymerization and grafting of monomers like acrylonitrile (AN) generate a precursor of acrylic fibers used in the preparation of starch compounds plus polymer (starch-graft-PAN), which are also biodegradable (Tharanathan, 2002).
Research on biodegradable plastics based on starch began since 1970 and currently continue in several laboratories throughout the world. Technologies that are still being developed, are related to the incorporation of the starch granule or starch in gelatinized form to formulations of the films manufactured during processes of compression, extrusion blowing, single or double-screw extrusion, and injection molding (Blacido et al., 2005; Parra et al., 2004). The problem presented by the films manufactured with starch is the sensitivity to humidity, which has been reduced by using in the formulations polyvinylalcohol (PVA), glycerin, sorbitol, nitrogenous bases, etc. (Shamekin et al., 2002; Smits et al., 2003; Finkenstadt and Willett, 2004; Yu, 2004; Acosta et al., 2006). Plasticization of the native starch granule or hydrolyzed starch is obtained through the structural disruption resulting from a decrease of the crystals during the extrusion process and by the action of the plasticizer, with a new type of material emerging known as thermoplastic starch (TPS) (Acosta et al., 2005; Villada, 2005). Likewise, studies have been conducted on TPS made from amylose and amylopectin; in these the barrier properties were analyzed, which showed high permeability to O2 and decrease of water vapor in TPS from amylose compared to those elaborated from amylopectins (Forssell et al., 2002; Dole et al., 2004; Jansson and Thuvander, 2004; Blacido et al., 2005).
TPS is a material obtained through the structural disruption (modification) given within the starch granule when it is processed with low water content and the action of thermal and mechanical forces in the presence of plasticizers that do not evaporate easily during processing (Bastioli, 2001). TPS presents several attributes, besides its biodegradability, it is a flexible and renewable material and it can be easily adapted to different processes of thermoplasticization by using standard equipment used in the manufacture of synthetic polymers, like injection molding, extrusion blow molding, injection molding, molding by compression, extrusion of flat film, and radiation by molding (Van Soest et al., 1996b; Zhai et al., 2003).
Adding natural polymers like starch inside the polyethylene (synthetic polymer), in granular for between 6 and 30%, is another approach in the manufacture of biodegradable packaging. Films from starch and low-density polyethylene (LDPE) contain up to 30% starch, shown as a partially biodegradable material. Another application of starch is the combination in gelatinized form in proportions between 30 and 70% mixed with synthetic polymers likewise gelatinized, like the case of polyvinylalcohol in proportions varying between 10 and 20% (Muratore et al., 2005).
Different products exist in the market made of synthetic polymers and gelatinized starch commercialized by Mater-Bi® (Hanna, 2004). However, currently both types of materials cannot be considered completely biodegradable compounds (Pedroso and Rosa, 2005). Plastic bottles of starch formed from foams through mixtures of starch with polylactic acid are used as filler material that dampens and protects against blows and vibrations during transport (Peesan et al., 2005; Xu et al., 2005).
In spite of the advantages of materials derived from starch, TPS presents little stability when humidity conditions are high (Avérous and Frigant, 2001; Avérous and Boquillon, 2004; Avérous et al., 2003). One of the problems of using TPS in bioplastics is its fragile nature, relatively caused by its low vitreous transition temperature (Tg) and the lack of sub-Tg relaxation due to the starch's molecular chain (Kenshi et al., 1999; Shogren et al., 1993). Additionally, eventual migration of plasticizers into the environment increases the material's fragility (De Graaf et al., 2003). Fragility is a problem of structural stability that increases over time due to diminished free volume and the retrogression of starch (Kuakoon et al., 2003). To increase TPS flexibility, a wide variety of plasticizers have been used like sugar, polyalcohols, amino acids, lipids, sorbates and phosphates (De Graaf et al., 2003; Fama et al., 2005; Kuakoon et al., 2003; Nashed et al., 2003; Parra et al., 2004; Petersson and Standing, 2005). Some studies hold that nitrogenous compounds prevent retrogression in starch gels longer than other additives, increasing the stability of the gel (Ma and Yu, 2004; Shogren et al., 1993). However, most nitrogenous compounds are in solid state and melt at high temperatures yielding little flexibility (Avérous and Frigant, 2001). Another problem in the development of TPS is the presence of high contents of amylose, which diminishes flexibility compared to TPS made from high contents of amylopectin (Van Soest and Essers, 1997). Additionally, during storage TPSs made from native starches undergo structural changes, presenting greater fragility or rigidity depending on plasticizer content (Van Soest et al., 1996a).
To summarize, against ordinary plastic polymers, thermoplastic starch presents disadvantages like: its solubility in water, high hygroscopicity, rapid aging due to retrogression, and poor mechanical properties, which limit some applications like packaging. These problems have been reduced when incorporating in the thermoplastic matrix natural fillers like cellulosic fibers that serve as reinforcement material to improve the mechanical properties: effort and elongation, properties of vital importance in evaluating any synthetic or biodegradable plastic material, given that they permit characterizing the material and its application in the development of any package (Salgado et al., 2008). These compound materials are comprised of three phases: reinforcement providing resistance and rigidity, the matrix that is the material sought to reinforce and the interface responsible for adequate compatibility between the matrix and the reinforcement (Tserki et al., 2005), where the quality of the interface determines the final properties of the material, permitting correct fiber-matrix adhesion and ensuring transference of stress from the matrix to the fiber (Tserki et al., 2006).
In relation to the development of biodegradable packaging from starch that include the use of high molecular weight polymers (100,000 to 400,000) for the formation of the molded polymer; patent US2007/0276317 reveals a biodegradable molded polymer comprising: 50 to 85% (p/p) of starch, 4 to 13% (p/p) of a polymer soluble in water, 10 to 35% (p/p) of sorbitol and other polyols like plasticizer, 0.5 to 10% (p/p) of polyethylene oxide or PEG, 0.25 to 3% (p/p) of a food grade emulsifier with an HLB of 2 to 10 and stearic acid between 0 and 1.5% (p/p).
In the same sense, U.S. Pat. No. 6,168,857 shows sheet-type compositions based on a matrix formed by starch and an organic polymer dispersible in water that incorporates organic or inorganic fibers dispersed homogenously (pine fibers), an inorganic mineral-type filler agent (between 30 and 60%), and a plasticizer. The document points to a procedure to elaborate a sheet that comprises the stages of mixing the components previously described, pass the mixture through a set of rollers at a temperature so that the organic polymer forms a layer that reduces adhesion of the material to the roller, heat to gelatinize the starch and remove a portion of water obtaining a sheet with a thickness less than 1 cm and a density greater than 0.5 g/cm3.
U.S. Pat. No. 5,939,192 refers to a thermoplastic material for use in the construction of vehicles and wooden articles like furniture, which comprises plant fiber particles distributed uniformly on a gel forming biopolymer matrix, where said fibers present a longitude between 0.05 and 35 mm and are impregnated with at least an acid resin. The formulation also includes a fat or oil, a polyol, and NaCl, where the gel forming biopolymer is selected from: starch, protein, lignin, hemicellulose, dextrin, pectin, or chitin and the fibrous material is selected from wood, cellulose, and cellulose obtained from recycled fibrous material. Additionally, it reveals a procedure to obtain the thermoplastic that comprises the stages of: (i) Heat the fibrous material at a temperature between 170 and 220° C. and add an acid resin or a mixture of said resin with an acid; (ii) prepare a mixture with the biopolymer, the fibrous material and the rest of the additives at a temperature above the room temperature and atmospheric pressure below environmental pressure until forming a gel where the fibrous material is partially impregnated; (iii) treat the mixture with a polyol; (iv) mold the mixture until forming a granulated material through extrusion free of pores at a temperature between 70 and 135° C. and pressure between 20- and 100 bar or by injection at a temperature between 110 and 210° C. and pressure between 250 and 1200 bar.
U.S. Pat. No. 7,067,651 divulges a process to prepare a biodegradable composition that comprises the stages of mixing starch, water, a plasticizer agent, a filler agent, and a compactant in the presence of alkali or salt until obtaining a uniform mixture and inject or compress this mixture in a closed mold at a temperature between 150 and 300° C. until gelatinization of the starch is reached. The composition is characterized for comprising: (i) starch at a percentage between 30 and 70% (p/p) prepared from cassava, rice, potato, corn, sorghum or sago alone or in mixtures in the native or modified form; (ii) a plasticizer agent at a percentage above 10% selected from: glycerol, sorbitol, glucose, fructose, syrup, honey, fatty acids, esters from fatty acids, mono-glycerides, di-glycerides, or fosfolipids alone or in mixtures; (iii) a filler agent selected from diatomaceous earth, bentonite, cassava pulp, natural fibers alone or in mixtures at a percentage above 30%; (iv) a compactant selected from: alginate, guar gum, carrageenan, konjak, soy protein, proteins of animal and plant origin and (v) an alkali or salt selected from CaOH, NaOH, KOH, CaCO3, Na2CO3, K2CO3, Na2HCO3, (NH4)2HCO3 alone or in mixtures to adjust the pH of the mixture to a neutral value or slightly alkaline.